1. Field of the Invention
This invention is in the field of testing devices, and is particularly concerned with a specimen which can be used to confirm stability of instrumentation used for ultraviolet scanning of test pieces in a non-destructive penetrant inspection process.
2. Description of the Prior Art
The penetrant method for determining surface discontinuities, utilizing a fluorescent penetrant material is being widely used in industry. Basically, the process consists in first cleaning the surface of the piece and then flooding it with a penetrant composition containing a fluorescent dye. The penetrant is permitted to dwell on the surface of the piece until the penetrant can become lodged within surface flaws. The excess penetrant is then wiped off or otherwise removed, leaving only the penetrant deposits which have found their way into surface and subsurface defects. Then, a wet or dry developer is applied to the piece to aid in extracting the entrapped penetrant from the flaws and making it more readily visible against the surface of the workpiece. The inspection of the piece for locating the flaws and determining the relative magnitude is normally done by an operator working in an inspection booth illuminated with ultraviolet or black light.
In more recent times, the fluorescent penetrant type inspection process has become more sophisticated with the introduction of scanning devices which direct ultraviolet light at the surface of a workpiece to be inspected, and a detector sensitive to fluorescent indications transforms the light impulses into electrical impulses which operate a recorder or the like. In order to provide meaningful results, however, this type of scanning equipment must be periodically calibrated against a suitable standard. There have been numerous such standards developed over the years, in most cases including a relatively thin steel plate which is coated with a thin, brittle layer of chromium plating. The plate is then bent around an arcuate surface to develop a series of fine cracks which can be detected by the fluorescent penetrant inspection process. Originally it was thought that such cracked plates could themselves be standards for calibrating the scanning equipment inasmuch as the location, length and depth of the cracks remain the same, but this has not proven to be the case. It was found, for example, that a given plate when tested by fluorescent penetrants would consistently give the same pattern of flaw indications but of substantially different intensities. In view of this fact, it is not possible to use cracked plates themselves as primary standards. The need still exists, therefore, for a standard which can simultaneously confirm the stability of the ultraviolet light level in the scanner, and the operation of the detector. Such a standard could be scanned before or after a test panel to provide confirmation of the validity of comparisons with previously run data. With a suitable primary standard, recorder deflection produced by the standard could be varied by adjustment in the distance of the ultraviolet light source or by varying the high voltage on the photo multiplier tube, should instrumentation shift be observed. The provision of a primary standard which accomplishes these purposes is the principal object of the present invention.